Add a quiver item, which is a lightweight container which can only store projectiles. The first projectile item placed inside determines what projectile is valid to put in after it (i.e. you cannot put arrows into a quiver followed by rocks for a sling). However, you can either empty the quiver, or put different types of projectiles into other ones. They can also hold stackable throwing weapons like darts or spears.

You can put a quiver in your quiver slot, and then using the #fire command will shoot ammo from the quiver in first-in-last-out order.